Update index documentation

AnnotationFileCitation wrongly references a file index instead of a text index. To match the url citation better, I suggest """The index at which to insert the file citation in the message.""". A more verbose string could give an example of inserting the citation.

https://github.com/openai/openai-python/blob/a8258744cbecf51321587fc870e8920bd2c07809/src/openai/types/responses/response_output_text.py#L28-L29

My interpretation is founded in a discussion and experimental observation that index lines up with the end of a word in the output_text. Note that for a "manual" check account for new lines as a single character and be aware that a text editor may not have a zero-indexed character position.

See also the file search guide. It is slightly ambiguous until tested that it refers to to an insertion point and not the location within the cited document.

Making a further observation within the same file, AnnotationFilePath has the same documentation for index, but it isn't relevant to my use case so I'm not sure how accurate it is.

Start at src/openai/types/responses/response_output_text.py around lines 28-29 and compare the index documentation for AnnotationFileCitation with the URL citation. Update the wording to describe the citation insertion point, then check whether the parallel AnnotationFilePath text also needs clarification; done means the affected documentation accurately describes index behavior.